Catalyst-coated membranes for water electrolysis

Abstract

A catalyst-coated membrane for a water electrolyser is provided. The catalyst-coated membrane comprises a thin film coating of an oxide of iridium on a first major surface of an electrolyte membrane. The thin film coating satisfies the following requirements: (i) the thin film coating has a first region adjacent to the electrolyte membrane and a second region distal to the electrolyte membrane; (ii) the porosity of the second region of the thin film coating is higher than the porosity of the first region of the thin film coating; and (iii) the thin film coating has a total porosity in the range of and including 20 to 60 vol%.

Description

[0001] CATALYST-COATED MEMBRANES FOR WATER ELECTROLYSIS

[0002] Field of the Invention

[0003] This invention relates to catalyst-coated membranes and membrane-electrode assemblies for water electrolysis. In particular, this invention relates to catalyst-coated membranes and membrane-electrode assemblies for proton exchange membrane (PEM) water electrolysis.

[0004] Background

[0005] Solid polymer electrolyte membranes, such as proton exchange membranes (PEMs) or anion exchange membranes (AEMs), may be employed for water electrolysis in combination with anode and cathode catalyst layers which are positioned on opposite sides of the membrane. In some cases, the anode catalyst layer and I or the cathode catalyst layer are applied to a face of the membrane to form a catalyst-coated membrane (CCM). In other cases, the respective catalyst layers may be applied to other components, such as transport layers, and the catalyst layers compressed against the membrane during assembly and subsequent use of the electrolysis cell.

[0006] Hydrogen evolution reaction (HER) catalysts are used in such electrolyser cathode catalyst layers, for example, HER catalysts comprising platinum, such as platinum on a carbon support. Oxygen evolution reaction (OER) catalysts are used in electrolyser anode catalyst layers, with iridium-containing catalysts, such as iridium oxide (IrOx), offering a particularly good balance between OER activity and stability under electrolysis conditions. Such catalysts are typically applied to membranes (or other components) in a catalyst ink and, after processing e.g. drying, form an adherent layer comprising the particulate catalyst and an ionconducting polymer.

[0007] Due to the relative scarcity and cost of iridium it is desirable to reduce the amount of iridium present in water electrolyser anode catalyst layers, for example to less than 1.0 mgircm-2. However, reducing the amount of iridium in the anode layer can lead to problems with limited catalyst layer conductivity and a reduction in the oxygen evolution efficiency of the anode layer.

[0008] It is also desirable to increase manufacturing efficiency of catalyst-coated membranes and other electrolyser components as the demand for hydrogen derived from water electrolysis rapidly increases in response to net zero targets.

[0009] It is known to use vapour deposition techniques to form an iridium oxide layer for water electrolysis. For example, it is described in Slavcheva, E. et al, “Sputtered iridium oxide films as electrocatalysts for water splitting via PEM electrolysis”, Electrochimica Acta 52 (2007) 3889 that sputtered iridium oxide films may be deposited on a titanium sublayer on Toray paper substrates. SEM images show that the lrC>2 layers are composed of uniform densely packed granular particles with a feature size of about 70 nm.

[0010] It is further described in Slavcheva, E. “Magnetron Sputtered Iridium Oxide an Anode Catalyst for PEM Hydrogen Generation” Macedonian Journal or Chemistry and Chemical Engineering, Vol 30., No. 1 , pp. 45-54 (2011) that thin films of iridium oxide may be deposited by reactive magnetron sputtering onto a titanium substrate on hydrophobic carbon paper. Catalyst layers manufactured using what the authors consider to be an optimal sputter regime (flow rate of oxygen gas 8-12 seem) are described as possessing a homogeneous microporous structure and are tested for electrochemical activity by hot pressing the layers onto Nation 117 membrane.

[0011] There remains a need to further enhance and develop anode layers for water electrolysers which address one or more previously identified challenges, in particular layers which provide high conductivity at low iridium loadings (< 1 mgircm-2), and which can be efficiently manufactured.

[0012] Summary of the invention

[0013] The present inventors have identified that thin film coatings with a graded porous structure offer particular utility as anode layers for water electrolysis. Such thin film coatings may be deposited directly onto an electrolyte membrane and provide lateral high conductivity, whilst maintaining OER catalytic activity at low iridium loadings.

[0014] Such thin film coatings may be advantageously deposited directly on the surface of an electrolyte membrane to form a catalyst-coated membrane. In such cases, the graded porosity of the thin film coating provides a lower porosity region adjacent to the membrane and a higher porosity region distal to the membrane. Such a configuration provides high lateral conductivity, whilst facilitating reagent transfer.

[0015] Therefore, in a first aspect of the invention there is provided a catalyst-coated membrane for a water electrolyser, the catalyst-coated membrane comprising a thin film coating of an oxide of iridium on a first major surface of an electrolyte membrane, the thin film coating satisfying the following requirements:

[0016] (i) the thin film coating has a first region adjacent to the electrolyte membrane and a second region distal to the electrolyte membrane;

[0017] (ii) the porosity of the second region of the thin film coating is higher than the porosity of the first region of the thin film coating;

[0018] (iii) the thin film coating has a total porosity in the range of and including 20 to 60 vol%. The thin film coatings may be advantageously incorporated into a membrane-electrode assembly. Therefore, in a second aspect of the invention there is provided a membraneelectrode assembly (MEA) for a water electrolyser comprising a catalyst-coated membrane and a transport layer, wherein the catalyst-coated membrane is according to the first aspect, and the MEA is configured such that the thin film coating on the surface of the electrolyte membrane is positioned between the electrolyte membrane and the transport layer.

[0019] In a third aspect of the invention there is provided a water electrolyser, such as a protonexchange membrane (PEM) water electrolyser, comprising a catalyst-coated membrane according to the first aspect, or a membrane-electrode assembly according to the second aspect.

[0020] Brief description of the Figures

[0021] Figure 1A and 1B show SEM images of cross sections of the CCM formed in Example 1A.

[0022] Figure 2A and 2B show SEM images of cross sections of the CCM formed in Example 1B.

[0023] Figure 3 shows an example analysis of the porosity of a first and a second region of the thin film coating of a CCM formed in Example 1A.

[0024] Detailed Description

[0025] Preferred and / or optional features of the invention will now be set out. Any of the preferred and / or optional features of any aspect may be combined, either singly or in combination, with any other preferred and / or optional features of any aspect of the invention unless the context demands otherwise.

[0026] The present invention provides thin film coatings comprising an oxide of iridium for use in a water electrolyser which comprises an electrolyte membrane, such as a proton exchange membrane (PEM) water electrolyser or an anion exchange membrane (AEM) water electrolyser. The thin film coatings act as the anode (oxygen evolving) catalyst during water electrolysis by facilitating the oxygen evolution reaction.

[0027] The term “thin film” takes its conventional meaning in the art, which will be understood by a skilled person. Suitably, the thin film coatings of the present invention have a thickness of no more than 2000 nm, typically no more than 1750 nm, or preferably no more than 1500 nm or no more than 1000 nm. The thin film coatings typically have a thickness of at least 60 nm, at least 100 nm, or preferably at least 200 nm. Accordingly, the thin film coatings of the invention may have a thickness in the range of and including 60 to 2000 nm, preferably 60 to 1750 nm, 100 to 1000nm, or 200 to 1000 nm. The thickness of the thin film coating may be measured by scanning electron microscopy (SEM). SEM analysis is carried out on cross sections of the structure and the thickness measured at multiple (for example 10) points. The thickness values are then determined by calculating the arithmetic mean of the measured values.

[0028] Typically, the thin film coating is provided by vapour deposition, i.e. the coating is a vapour- deposited thin film coating. Vapour deposition techniques, such as physical vapour deposition, for example a magnetron sputtering process, offer suitable control over the structure of the formed coating and are suitable for efficient large-scale production.

[0029] The thin film coating has a porous structure comprising, or consisting essentially of, an oxide of iridium. Suitably the oxide of iridium is an iridium oxide material, or a mixed iridium metal oxide, for example a metal oxide material comprising iridium and metal M, wherein M = Ta, Nb, Ti, Rh, Ru, or Pt. Such materials may be doped with one or more further elements or may be undoped. It may be preferred that the oxide of iridium is a metal oxide material comprising iridium and ruthenium. Such materials offer high oxygen evolution catalytic activity. It may be preferred that the oxide of iridium is a metal oxide material comprising iridium and platinum. Such materials offer high layer conductivity.

[0030] Preferably, the porous structure comprises, or consists essentially of, an iridium oxide (IrOx) material. Such iridium oxide (IrOx) materials may be amorphous, in which case the materials may comprise a mixture of oxide and hydroxide groups, with both Ir (III) and Ir (IV) species present, or may be crystalline, such as iridium (IV) oxide with a rutile crystal structure. Some I r(0) may be present in the porous structure, although it may be preferred that no lr(0) present, for example that no I r(0) is observable by x-ray diffraction analysis.

[0031] The thin film coating has a total porosity in the range of and including 20 to 60 vol%. It may be preferred that the thin film coating has a total porosity in the range of and including 30 to 50 vol%. The term “total porosity” as defined herein refers to the percentage of the total volume of the thin film coating that is not occupied by the oxide structure. The total porosity may be determined by SEM analysis of cross sections of the thin film coating. Parts of each image which correspond to pores or to the solid oxide structure may be classified and then the number of pixels in each region of interest counted and used to calculate the total porosity of the microstructure. Total porosity is measured at multiple (for example, 10) points.

[0032] The thin film coating advantageously has a graded porosity. The thin film coating has a first region adjacent to an electrolyte membrane, and a second region distal to the membrane. The thin film coating may consist of the first region and the second region. The thin film coating may have a third region between the first region and the second region.

[0033] The porosity of the second region of the thin film coating is higher than the porosity of the first region of the thin film coating. The relative porosity of the first and of the second region may be determined by SEM cross section analysis as described previously with regards to the total porosity. Providing a first region with a lower porosity provides a high lateral electronic conductivity to the thin film layer.

[0034] Suitably, the first region has a thickness in the range of and including 10 to 500 nm, preferably in the range of and including 50 to 300 nm. Suitably, the second region has a thickness in the range of and including 50 to 1500 nm, preferably in the range of and including 100 to 900 nm.

[0035] Preferably, the second region comprises dendrites of the oxide of iridium, or has a dendritic structure. Such a structure provides a high surface area and offers facilitation of gas bubble egress, whilst maintaining a degree of flexibility.

[0036] Preferably, the iridium loading is in the range of and including 0.05 to 0.70 mgircm-2, such as in the range of and including 0.10 to 0.70 mgircm-2, or 0.30 to 0.60 mgircm-2. The iridium loading of the thin film coating may be suitably determined by x-ray fluorescence (XRF) analysis.

[0037] Advantageously, the thin film coatings described herein offer high lateral conductivity. Preferably, the thin film coating has a sheet resistance less than 200 Q sq-1, less than 150 Q sq-1, or preferably less than 100 Q sq-1. The thin film coating may have a sheet resistance of greater than 20 Q sq-1, greater than 25 Q sq-1, greater than 30 Q sq-1, or greater than 35 Q sq_1. Preferably, the thin film coating has a sheet resistance in the range of and including 20 to 200 Q sq-1, 30 to 200 Q sq’1, 30 to 100 Q sq’1, or 40 to 100 Q sq’1.

[0038] The thin film coatings are provided on a surface of a polymer electrolyte membrane. Suitably, the electrolyte membrane is a proton-exchange membrane (PEM) or an anion-exchange membrane (AEM). Preferably, the electrolyte membrane is a proton exchange membrane. Such electrolyte membranes are formed from ion-conducting polymers, such as protonconducting polymers or anion-conducting polymers, such as a hydroxyl anion-conducting polymer. Such materials are known to those skilled in the art.

[0039] Suitably, the electrolyte membrane is a PEM and is formed from an ion-conducting polymer comprising sulfonic acid groups. Suitably, the ion-conducting polymer is a perfluorinated sulfonic acid ionomer, or a partially-fluorinated or non-fluorinated hydrocarbon sulfonic acid ionomer. Examples of suitable proton-conducting polymers include partially- or fully- fluorinated sulfonic acid polymers, such as perfluorosulfonic acid ionomers (e.g. Nation® (Chemours), Aciplex® (Asahi Kasei), Aquivion™ (Solvay Speciality Polymers), Flemion® (Asahi Glass Co.); or ionomers based on a sulphonated hydrocarbon such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products, Toyobo Corporation, and others. Suitably, the ion-conducting polymer has an equivalent weight of about 1100 or less, typically about 900 or less, suitably about 850 or less. Typically, the ion-conducting polymer has an equivalent weight of at least about 450. The electrolyte membrane may include additional components such as recombination catalysts, radical scavengers and reinforcement components. Recombination catalysts, such as platinum catalysts, for example Pt / C or platinum black, catalyse the reaction between hydrogen and oxygen and therefore help to reduce the cross-over of hydrogen through the membrane during electrolysis. Radical scavengers, such as oxides of cerium (for example CeC>2), can help to increase membrane durability.

[0040] Suitably, the reinforcing component is a porous polymer material, for example a microporous web or fibres of a polymer material, such as polyether ether ketone (PEEK), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene tetrafluoroethylene (ETFE), perfluoroalkyl alkane (PFA), or fluorinated ethylene propylene (FEP). For example, the planar reinforcing component may comprise electrospun PVDF or forcespun PVDF. In a preferred embodiment, the porous polymer material is expanded PTFE (ePTFE), for example, such as the microporous web structures of ePTFE supplied by Donaldson Company, Inc., known as Tetratex®, or supplied by other manufacturers. In other preferred embodiments, the reinforcing component can comprise a network of fibres (e.g. nanofibres), such as a network comprising polybenzimidazole (PBI) fibres, or a woven fabric, for example a woven fabric formed from PTFE thread. The network of fibres can be a non-woven mat of fibres (e.g. nanofibres), such as an electrospun mat of fibres or nanofibres.

[0041] Advantageously, the catalyst-coated membrane has restricted swelling in water at elevated temperatures. Such a restriction offers increased durability of the catalyst-coated membrane incorporating the thin film coatings. Such restricted swelling may be provided, for example, by the incorporation of multiple polymeric reinforcements into the membrane, or by using a polymeric reinforcement with high tensile strength, such as a woven fabric.

[0042] Therefore, in some embodiments, the electrolyte membrane has two or more reinforcing components, such as two or more layers of a microporous web of polymer, such as ePTFE. In some other embodiments the electrolyte membrane comprises a reinforcing component in the form of a woven fabric, such as a woven fabric formed from polymer threads, such as ePTFE or PEEK threads. Suitable materials are described in US11742507B2 (AGC INC).

[0043] Preferably, the electrolyte membrane has a thickness of less than or equal to 100 .m. It may be preferred that the membrane has a thickness of less than or equal to 95 .m, 90 .m, or 85 .m. It may be preferred that the membrane has a thickness of at least 10 .m, such as at least 15 .m, at least 20 .m, at least 25 .m, at least 30 .m or at least 40 .m. It may be further preferred that the membrane has a thickness in the range of and including 10 to 100 .m, such as 15 to 100 .m, 20 to 100 .m, 30 to 100 .m, 30 to 90 .m, or 40 to 90 .m. The membrane thickness may be measured by scanning electron microscopy (SEM). SEM analysis is carried out on cross sections of the membrane and the membrane and I or layer thickness measured at multiple (for example 10) points. The thickness values are then determined by calculating the arithmetic mean of the measured values. Typically, the SEM measurement is carried out on a cross section of the catalyst-coated membrane, which is embedded in resin, ground and polished.

[0044] The thin film coating is provided on the surface of the electrolyte membrane (the first major surface). The provision of the coating directly on the membrane offers a number of advantages with respect to provision of the coating on other components, such as a transport layer, including high manufacturing efficiency, low interfacial resistance, the ability to provide a uniform layer thickness in contact with the membrane, and allows for a variety of transport layers to be used. Furthermore, application of the thin film coating directly on the membrane enables the second region of the thin film to be positioned distal to the membrane, offering a facilitation of reagent transport to the catalyst surface.

[0045] It will be understood by the skilled person that the thin film coating may be present on the whole first major surface of the electrolyte membrane or may be present in one or more patches which correspond to the active area of the catalyst-coated membrane when incorporated into an electrolysis cell.

[0046] Typically, a cathode catalyst layer is provided on the second major face of the catalyst-coated membrane. Such cathode catalyst layers comprise a hydrogen evolution reaction catalyst, such as a platinum-based catalyst, for example platinum on a carbon support (Pt / C). Suitably, the cathode catalyst layer comprises a platinum catalyst and an ion-conducting polymer.

[0047] Separate film layers, typically formed from non-ion conducting polymers, may be positioned around the edge region of the CCM, for example on exposed surfaces of the ion-conducting membrane where no electrocatalyst is present (but will also often overlap on to the edge of the electrocatalyst layer) to provide a seal to prevent escape of reactant and product gases, to reinforce and strengthen the edge of the CCM and provide a suitable surface for supporting subsequent components such as sub-gaskets or elastomeric gaskets. An adhesive layer may be present on one or both surfaces of the seal film layer.

[0048] In a water electrolyser, additional transport layers are positioned each side of a membrane to facilitate reagent and product transfer to and from the catalyst layers, and to provide electrical contact. The catalyst-coated membrane and transport layer(s) are referred to together as a membrane electrode assembly (MEA). These additional transport layers may be known as porous transport layers or gas diffusion layers. These layers may or may not be directly attached to the CCM. Other components of a water electrolyser may include bipolar plates and current collector plates. Stacks of such assemblies make up an electrolyser system including power and control systems.

[0049] The MEAs of the present invention are configured such that the thin film coating on the surface of the membrane is positioned between the electrolyte membrane and a transport layer such that (a) it is in direct contact with the transport layer; or (b) it is in contact with an intermediate conductive layer positioned between the thin film coating and the transport layer.

[0050] Suitable transport layers at the anode side of the CCM are known to the skilled person and are typically formed from a metal-based porous structure. Such transport layers must be sufficiently conducting and in a form that is compatible with positioning adjacent to the CCM (without, for example, sharp edges or protrusions that would damage the membrane during use). Such metal-based porous structures may be in the form of, for example, felts or nonwoven cloths, mesh, foams and sintered compacts of metal-containing particles. For PEMWE applications, suitable PTLs comprise titanium. For AEMWE applications, suitable PTLs comprise nickel or stainless steel.

[0051] Suitable transport layers at the cathode side of the CCM are known to the skilled person and are typically non-woven papers or webs comprising a network of carbon fibres and a thermoset resin binder (e.g. the TGP-H series of carbon fibre paper available from Toray Industries Inc., Japan or the H2315 series available from Freudenberg FCCT KG, Germany, or the Sigracet® series available from SGL Technologies GmbH, Germany or AvCarb® series from Ballard Power Systems Inc., or woven carbon cloths. The carbon paper, web or cloth may be provided with a further treatment prior to being incorporated into as MEA either to make it more wettable (hydrophilic) or more wet-proofed (hydrophobic). The nature of any treatments will depend on the type of electrochemical device and the operating conditions that will be used.

[0052] The thin film coatings as described herein may be suitably formed by physical vapour deposition. The graded porosity of the thin film may be achieved using a magnetron sputtering process by selection of total chamber pressure, partial pressure of oxygen and pulsed DC power. For example, by flowing into a sputtering chamber Ar and O2 gases, and controlling pumping speed by way of a variable speed vacuum pump or a throttle valve, it is possible to achieve a controllable oxygen partial pressure maintained between 5 and 15 mTorr. Under these circumstances it is possible to create an iridium oxide microstructure with a zone of high porosity layered on top of a zone of lower porosity. Additionally, controlling power density can contribute to the synthesis of a film with open connected porosity. For example, maintaining a pulsed-DC power of 100 W on a 7.62 cm (3.0 inch) diameter cathode (13.1 W / cm2power density) while pulsing at a frequency of 100 kHz and a reverse period of 2 ps (duty cycle 80%) results in an average cathode voltage in the range of 250 - 400 V. These power settings in the aforementioned atmospheric condition can result in open microstructures with layered porosity.

[0053] The present invention will now be described with reference to the following examples, which are provided to assist with understanding the present invention and are not intended to limit its scope.

[0054] Examples

[0055] Test procedures

[0056] Scanning electron microscopy (SEM) analysis

[0057] Cross sections of the CCMs were analysed by SEM. The samples were analysed using a Zeiss Crossbeam 550 focussed ion beam / field emission electron microscope.

[0058] Compositional analysis and low-resolution general imaging: Accelerating voltage: 20kV; Aperture used: 30 - 60 micron; Working distance (WD): 7-8 mm; Detectors: Standard Secondary electron and standard backscattered electron detectors.

[0059] High-resolution low-accelerating voltage imaging: Accelerating voltage: 1 ,6kV; Aperture used: 20 - 30 micron; Working distance (WD): 2-3mm; Detectors: In-lens Secondary electron and In-lens backscattered electron detectors.

[0060] Determination of total porosity

[0061] Gray scale images were obtained from SEM analysis where the pixel intensity is proportional to the density and atomic number of the sample being scanned. In regions of interest, the pixel intensity was used to classify all the pixels associated with the membrane and pores of the thin film layer. Total porosity (vol %) of the thin film layer was determined by counting the number of pixels classified as a pore and the number of pixels classified as a solid IrOx region of the thin film. Total porosity is calculated as:

[0062] (Number of pixels in a pore region I total number of pixels (solid IrOx region + pore region)) x 100%

[0063] In-plane sheet resistivity measurement

[0064] The in-plane sheet resistivity of the catalyst-coated membranes was measured using a Loresta-GX MCP-T700 with a LSP probe from NH instruments. 5 measurements were made over 30 seconds with the average reported.

[0065] Electrochemical testing

[0066] CCMs were prepared with a Pt / C-PFSA ionomer cathode catalyst layer (with a Pt loading of 0.4 mg cm-2of Pt). The electrical performance of the CCMs was tested by the following method. The CCM was first conditioned with water flowing across the anode at 80 °C for 12 hours. Then the polarisation measurement was performed. Anode and cathode pressures were kept equal at atmospheric pressure. The current density was increased from 0 A / cm2to 1 A / cm2in steps of 0.04 A / cm2and then from 1 A / cm2to 4 A / cm2in steps of 0.08 A / cm2. The current density was then decreased from 4 A / cm2to 1 A / cm2in steps of 0.08 A / cm2and then from 1 A / cm2to 0 A / cm2in steps of 0.04 A / cm2. The upward going measurement (low to high current) was used for further analysis.

[0067] Preparation of Examples

[0068] Example 1- Preparation of a catalyst-coated membranes with a thin film lrOxcoating

[0069] Thin films of iridium oxide were deposited onto samples of a polymer electrolyte membrane (800EW PFSA, 80-micron thickness with two ePTFE reinforcements) by magnetron sputtering, a physical vapor deposition technique. Substrates were placed on a flat substrate fixture plate and masked with a metal sheet with perforated windows which allow for selective masking of the substrate. The fixture plate was loaded into a sputtering chamber and rotated underneath confocally arranged magnetrons mounted in a sputter-down configuration. The base pressure of the sputtering chamber was <5x10-7 Torr which was achieved with a Pfeiffer HiPace 1200 turbomolecular pump backed by at Edwards nXDS15i dry scroll vacuum pump.

[0070] Prior to deposition of thin films, substrates were plasma-etched in pure argon which flowed into the sputter chamber at 50 seem and was maintained at 10 mTorr by controlling downstream pumping speed by way of a throttle valve located just prior to the turbomolecular pump. The substrate fixture was plasma-etched by a Seren R601 RF generator running at 100 W for 5 minutes.

[0071] For deposition of the thin film, Ar and O2 gases were flowed into the sputter chamber and maintained between 5 to 15 mTorr. Cathodes were powered by pulsed-DC at a power of 100 W, a frequency of 100 kHz and a reverse period of 2.0 ps (duty cycle 80%) resulting in a cathode voltage between 250 to 400 V. The substrate fixture was rotated underneath the confocally arranged magnetrons at a distance of approximately 7.62 cm (3.0 inches). Film thickness was controlled by deposition time. The substrate fixture was maintained at room temperature (~21 °C) which was monitored by a thermocouple adjacent to the deposition zone.

[0072] Two catalyst-coated membrane samples were prepared with different iridium loadings as determined by X-ray Fluorescence (XRF) analysis:

[0073] Example 1A: 0.38 mgircm-2 Example 1 B: 0.54 mgircm-2

[0074] Comparative Example 2 (CEx2) - Preparation of a catalyst-coated membrane with a low loaded particulate IrOx coating

[0075] Samples of a polymer electrolyte membrane (800EW PFSA, 80-micron thickness with two ePTFE reinforcements) were coated using an ink comprising particulate IrOx and a mixture of PFSA ionomer in ethanokwater and then dried to form two catalyst-coated membrane samples with anode layers comprising particulate IrOx dispersed in ionomer which had iridium loadings as determined by X-ray Fluorescence (XRF) analysis of:

[0076] Comparative Example 2A: 0.34 mgircm-2

[0077] Comparative Example 2B: 0.55 mgir cm-2

[0078] Results

[0079] Figure 1A and 1 B shows SEM images of a cross section of the catalyst-coated membrane formed in Example 1A and Figure 2A and Figure 2B shows SEM images of a cross section of the catalyst-coated membrane formed in Example 1 B. These images show that in each case the formed thin film of IrOx is porous, with a graded porosity. Lower porosity is observed in the region of the thin film coating adjacent to the membrane (which is positioned at the bottom of the image). The region distal to the membrane has a dendritic microstructure.

[0080] Figure 3 shows an example analysis of the porosity of the first region and the second region of the thin film coating of Example 1A. The first region was determined to have a porosity of 25 vol% and the second region was determined to have a porosity of 35 vol%.

[0081] Table 1 provides a summary of test results generated on Examples 1A and 1 B and Comparative Examples 2A and 2B. The data shows that the thin film IrOx coatings provide a significantly lower sheet resistance than comparative examples, whilst maintaining high layer porosity and OER activity.

Claims

Claims1. A catalyst-coated membrane for a water electrolyser, the catalyst-coated membrane comprising a thin film coating of an oxide of iridium on a first major surface of an electrolyte membrane, the thin film coating satisfying the following requirements:(i) the thin film coating has a first region adjacent to the electrolyte membrane and a second region distal to the electrolyte membrane;(ii) the porosity of the second region of the thin film coating is higher than the porosity of the first region of the thin film coating;(iii) the thin film coating has a total porosity in the range of and including 20 to 60 vol%.

2. A catalyst-coated membrane for a water electrolyser according to claim 1 , wherein the thin film coating has a thickness in the range of and including 60 to 2000 nm.

3. A catalyst-coated membrane according to claim 1 or claim 2, wherein the second region comprises dendrites of the oxide of iridium.

4. A catalyst-coated membrane according to any one of the preceding claims, wherein the thin film coating has total porosity in the range of and including 30 to 50 vol%.

5. A catalyst-coated membrane according to any one of the preceding claims, wherein the thin film coating has an iridium loading in the range of and including 0.05 to 0.70 mgircnr2.

6. A catalyst-coated membrane according to any one of the preceding claims, wherein the thin film coating has a sheet resistance in the range of and including 30 to 100 Q sq-1.

7. A catalyst-coated membrane according to any one of the preceding claims, wherein the electrolyte membrane has a thickness less than or equal to 100 .m.

8. A catalyst-coated membrane according to any one of the preceding claims, wherein the electrolyte membrane comprises two polymeric reinforcement components.

9. A catalyst-coated membrane according to any one of the preceding claims, wherein the electrolyte membrane comprises a polymeric reinforcement component in the form of a woven fabric.

10. A catalyst-coated membrane according to any one of the preceding claims, wherein a cathode catalyst layer comprising a hydrogen evolution reaction (HER) catalyst is on the second major surface of the electrolyte membrane.11 . A membrane-electrode assembly (MEA) for a water electrolyser comprising a catalyst- coated membrane and a transport layer, wherein the catalyst-coated membrane is according to any one of claims 1 to 10 and the MEA is configured such that the thin film coating on the surface of the electrolyte membrane is positioned between the electrolyte membrane and the transport layer.

12. A water electrolyser comprising a catalyst-coated membrane according to any one of claims 1 to 10, or a membrane-electrode assembly according to claim 11.

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